.. If I am not mistaken Detroit has one of the highest murder rates in the USA and has a very high amount of gang activity which would demand more attention from law enforcement than say a place like Providence.
Detroit has figured out a way to avoid detection by law enforcement that the others have overlooked, at least for the time being.
I have no strong opinion on the number of formal members of what remains of the Detroit Family. In some very real aspects I don't think it matters so much to the victim of an extortion as to whether the criminal doing the deed has gotten his varsity letter or not.
But the point about non-Mafia violence in Detroit proper and some surrounding areas is an excellent one. It's not even gang violence so much as it is just stupid people with guns. In any event this keeps local Detroit police very very busy.
There's also enough non-mob corruption and theft in SE Michigan that I think the FBI would be kept busy investigating/prosecuting that. You combine that with a local (small) mob family that has always kept its head down, compared to the East Coast organizations, and it's not too surprising that to the extent they still exist as a formal organization, there's not a tremendous level of federal interest.
This is not to say they don't still have interests. A few of the local waste disposal/towing companies have some rather interesting back stories.